1. Field of the Invention
This invention relates to transducers for reading data from a magnetic recording medium and more particularly to transducers employing spin valve sensors.
2. Description of Related Art
A spin-valve is a type of magnetoresistive sensor for retrieving previously recorded magnetic signals from a magnetic recording medium, such as a disk or a tape surface. It is considerably more sensitive than the Amorphous MagnetoResistive (AMR) sensor. The sensing area of the spin-valve comprises of at least one very thin electrically conductive, non-magnetic spacer-layer sandwiched between two thin, electrically conductive magnetic layers. In each magnetic layer, the resultant magnetic field is characterized by its direction. The electrical resistance of the sensing area is affected by the angle between the magnetic fields in the two magnetic layers.
In a conventional spin-valve, the magnetic field in one of the magnetic layers is held (pinned) in a fixed direction. This magnetic layer is often called the "pinned-layer". Pinning can be achieved by exchange-coupling with a anti-ferromagnetic film. Alternatively, it can also be achieved by means of a permanent magnet. The magnetic film in another magnetic layer is allowed to rotate, subject to the "readback" magnetic field, which is the field from a previously recorded magnetic pattern. This magnetic layer is often called the "free-layer". Rotation of the magnetic field in the free-layer alters the angle between two fields separated by the spacer-layer, hence causing a resistance change in the sensing area.
It is desirable that the resistance change be proportional to the readback magnetic field. This feature is called the linearity of the sensor. To maximize the linearity, the magnetization in the free-layer and the pinned-layer are often set orthogonally to each other. It is also desirable that the sensor be robust against external fields, including the readback field. For this purpose a longitudinal bias field is often applied on the free-layer, usually through electrical leads of the sensor. This bias field can be supplied either by a pair of permanent magnets, or a pair of magnetic layers coupled to anti-ferromagnetic layers.
It is difficult to achieve the perpendicularity in the magnetization between the pinned and the free layers. Materials with distinct blocking temperature are often needed to provide desired magnetic field in distinct directions. In order to satisfy this constrain, one or more material with undesirable characteristics, such as chemical instability, must be selected. The resulting spin-valve sensor becomes very easily corroded. This makes the sensor difficult to process, and unreliable to use.
It is also difficult to maintain the perpendicularity between the magnetic fields, especially when the sensor is subject to an undue large current. This current is often induced by electrostatic discharge (ESD). The current heats up the sensor beyond the blocking temperature of exchange interface. It also generates an inductive magnetic field. As a result the magnetization of the pinned-layer may be altered inadvertently. This makes the spin-valve sensor difficult to handle and process. In some cases the flash temperature in an operating disk drive (caused by the slider-disk interference) is sufficiently high that the pinned-layer may be magnetically disturbed by the readback field. To prevent the disturbance, a conventional spin-valve is often forced to operate substantially farther away from the recorded medium than an inductive or AMR (Amorphous MagnetoResistive) sensor.
Thus a conventional spin-valve sensor is both difficult to process and unreliable to operate. To overcome this problem, U.S. Pat. No. 5,301,079 of Cain et al. for "Current Biased Magnetoresistive Spin Valve Sensor" describes a spin-valve in which both magnetic layers are free-layers. The magnetic layers are biased by the sensing current which flows orthogonally to the storage medium. The bias fields in two free-layers are both perpendicular to the readback field, but opposite to each other. Thus the readback field affects the angle between the resultant fields in the free-layers. This design works in principle. However it is difficult to fabricate.
U.S. Pat. No. 5,666,248 of Gill for "Magnetization of Pinned and Free Layers of Spin Valve Sensor Set by Current Fields" describes a spin-valve in which the sensing current flows in parallel with the medium, much the same as in conventional AMR (Amorphous MagnetoResistive) heads. Two magnetic layers are biased by the sensing current, similar to the bias scheme of a conventional AMR (Amorphous MagnetoResistive) head. However, Gill U.S. Pat. No. 5,666,248 states that an AMR (Amorphous MagnetoResistive) bias scheme will not work for a spin-valve, because the demagnetization field induced on the free-layer by the pinned layer is greater than the bias field from the sensing current. To minimize the demagnetization field, two flux guides are placed along upper and lower edges of the sensing area. However the flux guides are difficult to fabricate within an extremely small spin-valve (about 1 .mu.m.times.1 .mu.m in the sensing area).
U.S. Pat. No. 5,828,531 of Gill for "Spin Valve Sensor with Enhanced Magnetoresistance" describes a pinned magnetoresistive layer of soft ferromagnetic cobalt formed over an antiferromagnetic layer of FeMn, NiO or NiMn formed over and Al.sub.2 O.sub.3 gap.